Many conventional construction machines such as bulldozers are equipped with a hydraulic transmission system in which a hydraulic motor is connected, through a closed loop hydraulic circuit, to a variable displacement hydraulic pump driven by an engine and the hydraulic motor is driven by pressure oil discharged from the hydraulic pump to rotate wheels by the output rotation of the hydraulic motor. Such a hydraulic pump is constructed as follows. Output voltage is input to a controller in accordance with the position of an electric operating lever, an output signal from the controller is sent to a servo electromagnetic valve to switch the servo electromagnetic valve, and according to switching of the servo electromagnetic valve, the angle of a swash plate is adjusted.
The hydraulic transmission system described above is designed such that when the operating lever is placed in its neutral position, the angle of the swash plate of the variable displacement hydraulic pump becomes zero, with the swash plate held at its neutral position. In reality, however, the relationship between a servo command current value i and servo displacement D is as shown in FIG. 10 according to which when the servo command current value i falls within the range i.sub.B &lt;i&lt;i.sub.A, the position of the pump servo is within the undefined region .delta. and the discharge of the hydraulic pump is released to either the servo A or the servo B in a minute amount. This leads to an undesirable situation in which the hydraulic motor is slightly rotated, even if the operating lever is in the neutral position. This is a problem inherent to closed loop hydraulic circuits. In cases where the above hydraulic motor is used as a steering motor, if such a phenomenon occurs, the vehicle will turn during straight-ahead driving. In cases where the above hydraulic motor is used as a driving motor, an occurrence of the phenomenon causes the vehicle to move during stop.
As attempts to solve the above problem, there have been conventionally proposed methods for detecting the neutral position of the swash plate of the hydraulic pump and methods for controlling the pump swash plate to be kept at the neutral position. Examples of the methods are as follows.
(1) The rotation of a hydraulic motor is sensed and a pump swash plate is constantly controlled so as to make the rotation of the hydraulic motor be zero.
(2) A shim or the like is put in a servo control valve for controlling a hydraulic pump, thereby mechanically controlling the valve.
(3) A bypass valve for providing connection across a closed loop hydraulic circuit is used and controlled to be open by means of a neutral position signal.
(4) To automatically detect the neutral position of the hydraulic pump, the swash plate is displaced until the pressure on the pump port becomes equal to a preset value. The values of displacement of the swash plate are collected to obtain their average and this average is used as a signal value indicating the neutral position of the hydraulic pump. (Japanese Patent Publication (KOKOKU) Gazette No. 62-56384(1987))
(5) The differential pressure across a port is detected. If this differential pressure is more than or equal to a specified value, the pump swash plate is adjusted in a direction to make the differential pressure be zero so that the pump swash plate is placed at its neutral position. (Japanese Patent Publication (KOKAI) Gazette No. 63-214557)
The method (1) is costly and complex in structure, because it disadvantageously requires an expensive motor rotation sensor capable of detecting low-speed rotation of a motor. In addition, this method is designed to constantly control the pump swash plate without changing modes and therefore there is the possibility of faulty operation in the event of a failure. In the method (2), mechanical adjustment is necessary, and therefore lots of skill and man-hour are required when there arises a need for re-adjustment. Another disadvantage of the method (2) is that the influence of the backlash and hysteresis of the servo control valve when it is in the neutral position cannot be avoided. The method (3) requires a bypass valve operable based on a neutral position signal and such a bypass valve must have a complicated and large valve structure. It is conceivable to solve this problem by employing a restrictor which is in an open state at all times, but this arrangement entails decreased circuit efficiency. The method (4) is complicated in system construction as well as in operation, because it is designed to detect the neutral position of the hydraulic pump by detecting the displacement of the swash plate a plurality of times and averaging detection values. The method (5) is similar to the method (1) in that the pump swash plate is constantly controlled without changing modes, so that the method (5) is also liable to faulty operation in the event of a failure.
The present invention is directed to overcoming the foregoing problems and a primary object of the invention is therefore to provide a hydraulic driving circuit controlling method and its associated system which are simple in structure, easy to carry out neutral adjustment and capable of manual adjustment so that they can cope with a problematic situation in the event of a failure.